Making an Ohmic contact to a semiconductor can be challenging. One factor for controlling the contact resistivity is to ensure that the interface (e.g., between the metal and the semiconductor material) is as clean as possible. Thus proper surface cleaning and preparation steps can be implemented prior to depositing the metal over the semiconductor.
Another factor for improving the contact resistivity is to increase the amount of doping in the semiconductor material. Doping can be used to narrow the Schottky barrier that is formed at the metal-semiconductor interface so carriers can tunnel through the barrier. Doping of III-V materials can, however, present some notable challenges. Namely, when doping a III-V material there is generally a preference as to whether the dopant substitutes a group III atom or a group V atom in the material. However, since III-V materials contain both group III and group V elements, it is not always easy to predict which atoms (group III or group V) the dopant will replace.
For instance, most commonly used n-type dopants for III-V materials systems are group IV elements such as silicon (Si) and tin (Sn). The group IV dopants are known as amphoteric dopants since they can occupy a group-III site and provide an electron (i.e., an n-type dopant) or they can occupy a group-V site and “give a hole” (i.e., a p-type dopant). Considering the example of the III-V material indium gallium arsenide (InGaAs), at low doping levels Si will be an n-type dopant (occupying group-III sites). However, as more Si is added to the InGaAs, the Si atoms will also populate group-V sites and auto-compensate (counter dope) the InGaAs. As a result, it is experimentally observed that at doping levels of about 1×1019 to 3×1019 cm−3, the carrier concentration is saturated even if more Si (chemical doping) is added to InGaAs.
Thus, III-V semiconductor material growth techniques that reduce auto-compensation would be desirable.